- a drill string for drilling a borehole comprising a drill bit at its end,
- means for pumping drill mud down the borehole inside the drill string through the drill bit, and up the borehole on an outside of the drill string,
- a riser surrounding the drill string above the borehole,
- an annulus enclosed between the riser and the drill string forming a passageway for drill mud rising from the bottom of the borehole on the outside of the drill string,
- means for measuring a level of the drill mud inside the annulus inside the riser above the borehole, comprising:
  - a first pressure measurement device for measuring a first pressure prevailing inside the annulus at a first height and a second pressure measurement device for measuring a second pressure prevailing inside the annulus at a second height,
  - means for determining a density of the drill mud based on a ratio of a pressure difference between the first pressure and the second pressure and a product of a height difference between the first and the second height and the constant of gravity, and
  - means for determining the level based on a ratio of the a first pressure and a product of the density and the constant of gravity and/or based on a ratio of the second pressure and a product of the density and the constant of gravity.

According to a first preferred embodiment, the first and the second pressure measurement device are located below a mud return line, allowing for drill mud rising inside the annulus to flow out of the annulus.

According to a second preferred embodiment the rig comprises a monitoring unit for monitoring the level and detecting an abnormal mud backflow, in case the monitored level changes by more than a predetermined threshold.

According to a refinement of the second preferred embodiment, the monitoring unit monitors the density and detects an abnormal mud backflow, in case the monitored density changes by more than a predetermined threshold.

The invention further comprises a drilling rig according to the second preferred embodiment or the refinement thereof, wherein

- the monitoring unit comprises an internal clock, and
- the monitored levels and/or the monitored densities are monitored as a function of time.

The invention further comprises a method of operating a drilling rig according to the invention, comprising the steps of:

- monitoring the level (L) determined by the means for measuring the level (L), and
- detecting an abnormal drill mud backflow in case the monitored level (L) changes by more than a predetermined threshold.

The invention further comprises a refinement of the method according to the invention, comprising the steps of:

- detecting an abnormal drill mud backflow indicative of an onset of a potential blow out, in case the monitored level rises by more than a predetermined threshold, and/or
- detecting an abnormal drill mud backflow indicative of a loss of drill mud, in case the monitored level drops by more than a predetermined threshold.

The invention further comprises a refinement of the method according to the invention or the refinement thereof, comprising the steps of:

- monitoring the densities determined by the means for determining the density, and
- detecting an abnormal drill mud backflow in case the monitored density changes by more than a predetermined threshold.

A further refinement of the last mentioned method further comprises the step of detecting an abnormal drill mud backflow indicative of an onset of a potential blow out, in case the monitored density drops by more than a predetermined threshold.

It is an advantage of the invention, that the level of the drill mud inside the riser provides information, which may be used to detect abnormal drilling mud backflow. By monitoring the level, abnormal drilling mud backflows indicative of an onset of a threatening blow out or indicative of a loss of drill mud can be easily detected.

According to the invention, the required pressure measurement devices and the measurement electronic are located above the borehole. This has the advantage, that they can be easily installed, connected up and are easily accessible whenever necessary. It is not necessary to lower measurement devices down the bore hole, or to transfer measurement results up to the surface.

The invention and further advantages are explained in more detail using the FIGURE of the drawing, in which one exemplary embodiment is shown.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows: a drilling rig.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of drilling rig for drilling a well, in particular an oil or gas well. The rig comprises adrill string1 for drilling aborehole3 and adrill bit5 located at the end of thedrill string1.

Thedrill string1 is hollow, and means7 are foreseen for pumping drill mud down thedrill string1 through thedrill bit5 into theborehole3. These means7 comprise e.g. a pump for pumping drill mud through asupply line9 connected to thedrill string1. From the bottom of theborehole3, the drill mud will then return up theborehole3 on an outside of thedrill string1. Above the borehole3 ariser11 is foreseen, which encloses a section of thedrill string1, which is located above theborehole3. Anannulus13 enclosed between theriser11 and thedrill string1 forms a passageway for drill mud returning from the bottom of theborehole3 on the outside of thedrill string1, through which the returning drill mud rises up.

According to the invention, the rig comprises measurement means for measuring a level L of the drill mud inside theannulus13 inside theriser11 above theborehole3.

These means comprise a firstpressure measurement device15 for measuring a first pressure p₁, prevailing inside theannulus13 at a first height H₁above theborehole3 and a secondpressure measurement device17 for measuring a second pressure p₂prevailing inside theannulus13 at a second height H₂above theborehole3. The

pressure measurement devices

15,17 comprise e.g. a first pressure sensor, installed in an opening foreseen in theriser11 at the first height H1, and a second pressure sensor, installed in an opening foreseen in theriser11 at the second height H2.

In addition the means for determining the level L of the drill mud inside theriser11 comprise a measurement electronic19 connected to the first and the second

pressure measurement device

15,17

Both

pressure measurement devices

15,17 are located below amud return line21, allowing for drill mud rising inside theannulus13 to flow out of theannulus13. The drill mud flowing out of theannulus13 is supplied to amud recirculation system23 designed to separate the drill cutting contained in the drill mud from the drill mud and to recondition the drill mud, which can then be resupplied to thedrill string1 via thesupply line9.

During operation of the rig, the first and the second pressure p₁, p₂are measured by the first and the secondpressure measurement device15,17, and the measurement electronic19 determines the density
of the drill mud based on a pressure difference Δp=p₁−p₂between the first pressure p₁and the second pressure p₂and a product of a height difference ΔH=H2−H1 between the first height H1 and the second height H2 and the constant of gravity g. The density
is given by:
$ρ = \frac{p_{1} - p_{2}}{g Δ H}$
Based on the determined density
and the first or the second pressure p₁, p₂a density compensated the level L of the drill mud inside theannulus13 of theriser11 is determined. To this extend, the measurement electronic19 determines the level L based on the first pressure p₁and a product of the density
and the constant of gravity g and/or based on the second pressure p₂and a product of the density
and the constant of gravity g. In either case, the level L corresponds to the position of asurface25 of the drill mud inside theannulus13 which can e.g. be determined as a distance D1, D2 between thesurface25 and the height H1, H2 at which the respective pressure p₁, p₂is measured.
The level L corresponding to the position of asurface25 of the drill mud inside theannulus13 can be determined as a distance D1 between thesurface25 and the first height H1 at which the first pressure p₁is measured, given by:
$D 1 = \frac{p_{1}}{ρ * g}$
and/or it can be determined as a distance D2 between thesurface25 and the second height H2 at which the second pressure p₂is measured, given by:
$D 2 = \frac{p_{2}}{ρ * g}$
During normal operation of the rig, drill mud having a suitable density is supplied to the rig, and pumped down the borehole inside thedrill string1 with a constant pumping pressure. In consequence the drill mud returning to the surface will rise up to a level L, which remains fairly constant during normal operation, and will have a fairly constant density
.
Thus during operation of the rig, the level L of the drill mud inside theriser11 is monitored based on the level measurements performed by the level measurement means provided on the rig. Monitoring is performed by amonitoring unit27, which can be an integral part of the measurement electronic19, or a separate unit connected to the measurement electronic19. Themonitoring unit27 is designed to detect an abnormal drill mud backflow in case the monitored level L changes by more than a predetermined threshold.
Preferably themonitoring unit27 is designed to not only monitor the determined levels L, but also the determined densities
, and to detect an abnormal drill mud backflow in case the monitored density
changes by more than a predetermined threshold.
Themonitoring unit27 preferably comprises aninternal clock29, and the monitored levels L and the monitored densities are monitored as a function of time.
If a gas or a liquid under high pressure enters theborehole3, the pressure increase inside theborehole3 will exert an additional force onto the drill mud, pushing the drill mud up theborehole3 on the outside of thedrill string1. The additional pressure exerted by the gas or liquid influx, will thus cause the level L of the drill mud inside theriser11 to rise. In consequence an abnormal drill mud backflow indicative of the onset of a potential blow out will be detected by themonitoring unit27, in case the monitored level L rises by more than a predetermined threshold.
The increase of the level L as well as the speed at which it increases depends on the pressure exerted by the influx in relation to the counter pressure exerted by the drill mud, in particular the overhead pressure exerted by the mud column above the location of the influx, the type of influx and the consistency of the drill mud.
In most cases, higher pressures exerted by the influx will cause the level L to increase higher and faster. Depending on the situation at the location of the influx, in particular the pressures acting there, the type of influx and the consistency of the drill mud, it is very well possible, that the pressures exerted by the influx cause the level L of the drill mud to increase inside theriser11 well before drill mud containing the gas or liquid will return to the surface.
Gas or liquid contained in the drill mud reduces the density
of the drill mud. Thus gas or liquid contained in the drill mud reaching the section of theriser11 equipped with thepressure measurement devices15,17 can be detected based on the monitored density
. In consequence an abnormal drill mud backflow indicative of the onset of a potential blow out will be detected by themonitoring unit27, in case the monitored density
drops by more than a predetermined threshold.
In all cases where the level L inside theriser11 rises before the drill mud containing the gas or liquid returns to the surface, the level increase caused by the influx, will be detected before a change in density
caused by the influx will be detected. In these cases, an abnormal mud backflow, indicative of the onset of a threatening blow out, will be detected by the rig according to the invention earlier, than would be possible based on the density measurements.
In all other cases, monitoring the level L and monitoring the density
constitute two different methods of detecting abnormal mud backflow indicative of the onset of a threatening blow out, which are preferably applied in parallel, in order to enhance the probability of an early detection of the onset of a threatening blow out.
In case an abnormal drill mud backflow, indicative of an onset of a threatening blow out is detected corresponding safety measures, e.g. the activation of blow out preventers foreseen on the rig, are applied.
If thedrill bit5 hits a cavity, drill mud supplied through thedrill string1 to the bottom of the borehole will enter the cavity. In this case, the level L inside theriser11 will drop due to the amount of drill mud lost into the cavity. Correspondingly, themonitoring unit27 will detect an abnormal drill mud backflow indicative of a loss of drill mud, in case the monitored level L drops by more than a predetermined threshold. In case an abnormal drill mud backflow, indicative of a loss of drill mud is detected a corresponding warning or an alarm can be issued, and suitable safety measures, e.g. measures for sealing the borehole walls, can be applied.

1 drill string
3 borehole
5 drill bit
7 means for pumping drill mud
9 supply line
11 riser
13 annulus
15 first pressure measurement device
17 second pressure measurement device
19 measurement electronic
21 return line
23 mud recirculation system
25 surface of the drill mud
27 internal clock

Claims

1-9. (canceled)

10. A drilling rig for drilling a well, in particular an oil or gas well, comprising:

a drill string for drilling a borehole, comprising a drill bit at one end, means for pumping drill mud down the borehole inside said drill string through said drill bit, and up the borehole on an outside of said drill string;

a riser surrounding said drill string above the borehole;

an annulus enclosed between said riser and said drill string forming a passageway for drill mud rising from the bottom of the borehole on the outside of said drill string;

means for measuring a level of the drill mud inside said annulus inside said riser above the borehole, comprising:

a first pressure measurement device for measuring a first pressure prevailing inside said annulus at a first height and a second pressure measurement device for measuring a second pressure prevailing inside the annulus at a second height;

means for determining a density of the drill mud based on a ratio of a pressure difference between said first pressure and said second pressure and a product of a height difference between said first and said second height and the constant of gravity; and

means for determining said level based on a ratio of said first pressure and a product of the density and the constant of gravity and/or based on a ratio of said second pressure and a product of the density and the constant of gravity.

11. The drilling rig according toclaim 10, wherein:

said first and said second pressure measurement device are located below a mud return line, allowing for drill mud rising inside said annulus to flow out of said annulus.

12. The drilling rig according toclaim 10, further comprising:

a monitoring unit for monitoring the level and detecting an abnormal mud backflow, in case the monitored level changes by more than a predetermined threshold.

13. The drilling rig according toclaim 12, wherein:

said monitoring unit monitors the density and detects an abnormal mud backflow, in case the monitored density changes by more than a predetermined threshold.

14. The drilling rig according toclaim 12, wherein:

said monitoring unit comprises an internal clock; and

said monitored levels and/or the monitored densities are monitored as a function of time.

15. A method of operating a drilling rig, comprising the steps of:

monitoring the level determined by means for measuring the level; and

detecting an abnormal drill mud backflow in case the monitored level changes by more than a predetermined threshold.

16. The method according toclaim 15, comprising the steps of:

detecting an abnormal drill mud backflow indicative of an onset of a potential blow out, in case a monitored level rises by more than a predetermined threshold; and/or

detecting an abnormal drill mud backflow indicative of a loss of drill mud, in case the monitored level drops by more than a predetermined threshold.

17. The method according toclaim 15, comprising the steps of:

monitoring the densities determined by the means for determining the density; and

detecting an abnormal drill mud backflow in case the monitored density changes by more than a predetermined threshold.

18. The method according toclaim 17, comprising the step of:

detecting an abnormal drill mud backflow indicative of an onset of a potential blow out, in case the monitored density drops by more than a predetermined threshold.